The project is aimed at characterizing basic mechanisms of chemical communication between nerve cells. The information obtained should ultimately help in understanding the actions of psychotherapeutically useful drugs, and may possibly provide a basis for rational design of new therapeutic approaches. The work could also provide a better basis for understanding brain mechanisms of alcohol and drug addiction. One focus of our studies is the monoamine neurotransmitter serotonin (5-hydroxytryptamine, 5-HT). Serotonin has been implicated in the etiology of clinical depressive disorders and in disturbances of sleep. The experiments will concentrate on regulatory roles of intracellular calcium ion concentration ([Ca++]i) in both presynaptic and postsynaptic mechanisms of synaptic transmission. It is widely believed that [Ca++]i is important in both classes of mechanism, but difficulties in experimental measurement of [Ca++]i in living cells have limited the information available. New methods for measurement of [Ca++]i in isolated cells will be used in conjunction with electrophysiological, pharmacological, and optical means of manipulating and recording cellular activity. Two sets of specific questions will be addressed. The first concerns the role of [Ca++]i in excitation-secretion coupling at presynaptic terminals: (1) What happens to [Ca++]i during excitation? (2) Can kinetics of [Ca++]i explain the kinetics of transmitter secretion? (3) How does [Ca++]i promote secretion? The second set of questions to be addressed concerns the role of [Ca++]i in cellular responsiveness to 5-HT: (1) What happens to [Ca++]i during 5-HT action? (2) How does 5-HT act on [Ca++]i? (3) Does [Ca++]i mediate actions of 5-HT on ion channels? Measurements of [Ca++]i will be carried out by photometry of indicator dyes such as arsenazo III. Cells types to be studied include the ganglion cells of gastropod molluscs, squid giant neurons, and chromaffin cells from bovine (cow) adrenal glands.